Homologous recombination plays a critical role in reductional segregation of chromosomes in meiosis. Meiotic recombination is initiated by the programmed induction of DNA double strand breaks (DSBs) and involves a mechanism related to recombinational repair of DSBs in mitotic cells. The key difference between meiotic and mitotic recombination is that the meiotic process must occur between homologous chromatids and the mitotic process usually occurs between sister chromatids. The central step of recombination is homologous strand invasion and exchange. This process is catalyzed by recombinases that are structurally and functionally related to the bacterial strand exchange protein, RecA. There are two RecA-like recombinases in most eukaryotic organisms, including budding yeast and humans;Rad51 is the only RecA-like recombinase involved in mitotic recombination. Dmc1 is a meiosis-specific recombinase that can function in the absence of Rad51, but often cooperates with it. In order to promote strand invasion, recombinases must first polymerize into helical filaments on the tracts of single strand DNA (ssDNA) that form at sites of DSBs. Accessory factors, called mediators, allow recombinase to displace single strand DNA binding proteins as they form filaments. Recent evidence indicates that a second type of accessory factor expends energy to promote dissociation of recombinases from DNA using ATP hydrolysis dependent DNA-translocase activity. The proposed work seeks to elucidate meiotic recombination in budding yeast. The aims of this proposal are: 1. To visualize the architecture of the meiotic recombinosome using high-resolution light microscopy with the aim of determining if the previously detected side-by-side Rad51 and Dmc1 structures have a specific organization relative to the axes of the two chromosomes engaged in a recombination event. 2. To determine the mechanism underlying the functional differences between the DNA translocases Tid1/Rdh54 and Rad54. 3. To determine the mechanisms through which Dmc1's accessory factors interact to stimulate recombination. Defects in meiotic recombination cause chromosome non-disjunction and loss, both of which lead to birth defects and spontaneous abortion. Failure of recombinational repair in mitosis is implicated in the etiology of breast cancer and other malignancies. PUBLIC HEALTH RELEVANCE: We will characterize the recombination machinery that exchanges chromosomes arms during meiosis. Meiosis forms eggs and sperm. Understanding recombination may prevent birth defects caused by errors in meiosis. Recombination prevents cancer in humans, so this work may improve cancer prevention and treatment.